1. Field of the Invention
This invention relates to a display control apparatus and method which provides display control of a flat-panel display and a CRT display in portable electronic equipment, and more particularly to a display control apparatus and method in which unnecessary power consumption is eliminated when the CRT display is not available.
2. Description of the Related Art
In recent years, the computer industry has seen enormous growth in the market for portable personal computers that are commonly referred to as notebook and subnotebook personal computers. Generally, such portable computers incorporate a flat-panel display, and are connectable to a CRT display. The flat-panel display is made up of a plasma display (PDP), an STN monochrome liquid-crystal display (LCD), an STN color LCD, a TFT color LCD, or the like.
FIG. 1 shows a conventional display control apparatus, which corresponds to, for example, a Model WD80C22 manufactured by Western Digital Corporation. This display control apparatus is provided with a display controller 1 that performs display control concerning an analog color CRT display and a flat-panel display, and a color processing section 2 that performs color processing to drive the analog color CRT display by the control of the display controller 1. In the display control, the display controller 1 produces various display control signals such as 8-bit pixel video data VC7-0 corresponding to a pixel of display data serially read out from a video RAM, and a display clock signal DACLK synchronous to the pixel video data VC7-0. The color processing section 2 includes an (I/O) interface 3 connected to the display controller 1, a color palette RAM 4 that color-converts the pixel video data VC7-0 supplied via the interface 3 into a digital RGB signal, and a video digital-to-analog converter (DAC) 5 that converts the digital RGB signal supplied from the color palette RAM 4 into an analog RGB signal. The color palette RAM 4 has 256 memory areas that each store palette data formed of a set of 6-bit color component data DR5-0, DG5-0, and DB5-0 representing red, green, and blue levels, respectively. It outputs as a digital RGB signal a set of color component data DR5-0, DG5-0, and DB5-0 stored in the memory area selected by pixel video data VC7-0. In the video DAC 5, the digital RGB signal is processed by a sequence of digital-to-analog conversion performed in synchronism with the display clock signal DACLK supplied from the input/output interface 3. Specifically, the video DAC 5 latches a digital RGB signal in a first clock cycle, converts the latched digital RGB signal into an analog RGB signal in a second clock cycle, and outputs the analog RGB signal in a third clock cycle. The input/output interface 3 supplies the color component data DR5-0, DG5-0, and DB5-0 of the digital RGB signal output from the color palette RAM 4 to the display controller 1. The display controller 1 contains a flat-panel controller 6 that drives a flat-panel display according to the color component data DR5-0, DG5-0, and DB5-0. When the flat-panel display is a monochrome LCD, however, only the color component data DG5-0 is used to drive the monochrome LCD. In this case, the color palette RAM 4 serves as a converter that performs tone-conversion on the pixel video data VC7-0.
When key-in operation is detected to be absent in the computer system for a preset period of time, the display controller 1 supplies a power-down signal DACPD. For the video DAC 5 to the color processing section 2 to terminate the driving of the analog color CRT display. The video DAC 5 stops inputting of the digital RGB signal in response to the power-down signal DACPD.
However, operation of the video DAC 5 cannot be resumed normally in a case where it has been suspended by cutting off the display clock signal DACLK at the time when the power-down signal DACPD is supplied. In this case, it is not certain that the video DAC 5 enters into the least power consuming state very close to the state of fully cutting off the power supply. Further, the flat-panel controller 6 drives the flat-panel display according to the digital RGB signal supplied from the color processing section 2. Therefore, it seems difficult to further reduce the power consumption in the color processing section 2 when only the flat-panel display is driven.